Here is an interesting summary of the key points of a research paper entitled The Effect of Immigration on Productivity:Evidence from the US States by Giovanni Peri of the National Bureau of Economic Research.
Claire Brunel summarizes the findings thusly in a digest form of the paper:
1. Citizen hours and jobs are not taken by immigrant labor; there is an additive effect.
2. Immigrants increase the total factor productivity.
3. Immigrants decrease capital intensity.
4. Less educated citizens pay is minimally impacted, and higher educated citizens pay increases.
For more reading on these findings here is the digest form of the paper: http://www.nber.org/digest/mar10/w15507.html
Or for deeper reading visit this link to the actual paper: http://www.nber.org/papers/w15507